In an automotive steering system, for example, a cross joint is used for interconnecting a column shaft and a steering shaft. The cross joint includes: a cross shaft including four shaft portions projecting in four directions from a side periphery of a body portion thereof; a plurality of rollers rolling on outer peripheries of the shaft portions; and closed-end cylindrical bearing cups fitted on these rollers. Each of the above shafts is provided with a pair of opposing yokes at an end thereof. Each shaft portion of the cross shaft is axially rotatably supported by a bearing hole formed in each yoke via the bearing cup having the plural rollers arranged on an inner periphery thereof.
When transfer torque is increased in a joint structure including such a cross joint, the rollers may be caused to slip so that the joint structure may encounter backlash or pulsations due to torque transfer failure.
In order to solve this problem, the following structure, such as disclosed in Patent Document 1 (International Publication No. WO2003-064877), has been proposed in the art. The structure is constituted such that the rollers and the shaft portions of the cross shaft are in interference fit relation.
The joint structure including the cross joint according to Patent Document 1 is assembled as follows. In a state where a shaft portion 44 of a cross shaft 43 is inserted in a bearing hole 42 of a yoke 41, as shown in FIG. 7, a bearing cup 46 having plural rollers 45 arranged on an inner periphery thereof is moved toward the shaft portion 44 from place axially outwardly of the shaft portion 44. While the bearing cup 46 is press inserted into the bearing hole 42, the rollers 45 in the bearing cup 46 are fitted about the shaft portion 44 via a distal end thereof. This causes the bearing cup 46 to be decreased in diameter, so that the rollers 45 and the shaft portion 44 are brought into pressure contact with each other, or brought into the interference fit relation or a state where the rollers and the shaft portion define zero clearance therebetween.
However, the rollers 45 on the inner periphery of the bearing cup 46 may be brought into hard contact against an outside circumference edge (corner) 47 of a distal end face of the shaft portion 44 because the bearing cup 46 is decreased in diameter by the press-insertion into the bearing hole 42 when the rollers 45 in the bearing cup 46 are fitted about the shaft portion 44. This leads to fear that the outer periphery of the shaft portion 44 defining a rolling contact surface, and the rollers 45 may sustain damages (indentation) or deformation. Particularly at the initial stage of the fitting engagement when the rollers 45 come into contact with the distal end of the shaft portion 44, the distal end of the shaft portion 44 is susceptible to damages due to a great pressure on a contact surface between the rollers 45 and the shaft portion 44. If the rolling contact surface of the shaft portion 44 and the rollers 45 sustain damages in this manner, the joint structure may be decreased in performance and durability.